/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;

import java.io.Serializable;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An ordering that uses the natural order of the values.
 */
@GwtCompatible(serializable = true)
@SuppressWarnings({"unchecked", "rawtypes"}) // TODO(kevinb): the right way to explain this??
@ElementTypesAreNonnullByDefault
final class NaturalOrdering extends Ordering<Comparable<?>> implements Serializable
{
    static final NaturalOrdering INSTANCE = new NaturalOrdering();

    @CheckForNull
    private transient Ordering<@Nullable Comparable<?>> nullsFirst;
    @CheckForNull
    private transient Ordering<@Nullable Comparable<?>> nullsLast;

    @Override
    public int compare(Comparable<?> left, Comparable<?> right)
    {
        checkNotNull(left); // for GWT
        checkNotNull(right);
        return ((Comparable<Object>) left).compareTo(right);
    }

    @Override
    public <S extends Comparable<?>> Ordering<@Nullable S> nullsFirst()
    {
        Ordering<@Nullable Comparable<?>> result = nullsFirst;
        if (result == null)
        {
            result = nullsFirst = super.<Comparable<?>>nullsFirst();
        }
        return (Ordering<@Nullable S>) result;
    }

    @Override
    public <S extends Comparable<?>> Ordering<@Nullable S> nullsLast()
    {
        Ordering<@Nullable Comparable<?>> result = nullsLast;
        if (result == null)
        {
            result = nullsLast = super.<Comparable<?>>nullsLast();
        }
        return (Ordering<@Nullable S>) result;
    }

    @Override
    public <S extends Comparable<?>> Ordering<S> reverse()
    {
        return (Ordering<S>) ReverseNaturalOrdering.INSTANCE;
    }

    // preserving singleton-ness gives equals()/hashCode() for free
    private Object readResolve()
    {
        return INSTANCE;
    }

    @Override
    public String toString()
    {
        return "Ordering.natural()";
    }

    private NaturalOrdering()
    {
    }

    private static final long serialVersionUID = 0;
}
